1. Field of Invention
The invention relates to a drive system for hybrid vehicles, which drives wheels by using a combination of an internal combustion engine and electric motors.
2. Description of Related Art
As conservation of air environment and saving of fuel resources become increasingly important, hybrid vehicles whose wheels are driven by a combination of an internal combustion engine and an electric motor (or motors) have been attracting much attention in the field of automobiles. In the hybrid vehicles, wheels are driven in various manners by an internal combustion engine and an electric motor (or motors) arranged in various forms so as to achieve a wide variety of combinations of rotation speed and drive torque. Motor vehicles used to be driven by using only internal combustion engines. However, the development of hybrid vehicles in the automotive field was started by replacing a part of a drive system that conventionally uses only an internal combustion engine, with an electric drive system including, for example, an electric motor(s).
With this background, it is presently assumed that hybrid vehicles are capable of running only by use of an internal combustion engine. Japanese Laid-open Patent Publication No. 11-198669 discloses one example of a hybrid-vehicle drive system. In this hybrid-vehicle drive system, a first electric motor/generator is connected in series with a crankshaft of an internal combustion engine and a power shaft is arranged to be driven by one or both of the internal combustion engine and the first electric motor/generator serving as a motor. The power shaft and an output shaft of a second electric motor/generator are respectively connected to a ring gear and a sun gear of a planetary gear mechanism, thereby connecting both shafts to each other. A carrier of the planetary gear mechanism, which serves as an output shaft, is connected to a transmission, which in turn is connected to drive wheels. In the hybrid-vehicle drive system thus constructed, even when only the internal combustion engine operates as a driving motor, the drive system is able to provide a wide variety of operating or running modes required by the vehicle by utilizing the speed-ratio changing function of the transmission, as is the case with conventional vehicles using only the internal combustion engine. This may be considered as one typical example reflecting the origin of hybrid vehicles as described above.
On the other hand, there has been proposed another hybrid-vehicle drive system that eliminates the need of providing a transmission which was conventionally disposed between an output shaft of an internal combustion engine and a transmission. In this hybrid-vehicle drive system, an internal combustion engine and an electric motor (or motors) are combined to provide a driving source of a motor vehicle such that the motor serves to absorb a difference between the rotation speed of an output shaft of the internal combustion engine and that of vehicle axles. The difference is caused by deviation of the relationship between the rotation speed and the drive torque actually obtained by the internal combustion engine from that relationship required at the vehicle axles. FIG. 1 is a view schematically showing a construction of such a hybrid-vehicle drive system.
In FIG. 1, an internal combustion engine 1 is mounted in a vehicle body (not shown), and has an output shaft (or a crankshaft) 2. A planetary gear mechanism 3 includes a sun gear 4, a ring gear 5, planetary pinions 6, and a carrier 7. The crankshaft 2 is coupled to the carrier 7. A first electric motor/generator (MG1) 8 includes a coil 9 and a rotor 10. The rotor 10 is connected to the sun gear 4 while the coil 9 is supported on the vehicle body. One end of a propeller shaft 11 is connected to the ring gear 5. In the drive system thus constructed, the planetary gear mechanism 3 is operable to distribute power received from the internal combustion engine to the first motor/generator 8 and the propeller shaft 11 as a wheel-drive shaft. Thus, the planetary gear mechanism 3 serves as a power distribution mechanism. A second electric motor/generator (MG2) 12 is connected to an intermediate portion of the propeller shaft 11. The second motor/generator 12 includes a coil 13 and a rotor 14. The coil 13 is supported on the vehicle body. The rotor 14 may be connected to the propeller shaft 11 in any manner. In the drive system as shown in FIG. 1, for example, the rotor 14 is connected to the propeller shaft 11 such that a gear 16 supported and rotated by the rotor 14 engages with a gear 15 provided on the propeller shaft 11. The other end of the propeller shaft 11 is connected to a pair of vehicle axles 18 via a differential gear unit 17. Wheels 19 are attached to the respective vehicle axles 18.
In the drive system shown in FIG. 1, the crankshaft 2 rotates as a unit with the carrier 7, and the rotation speed of these components 2, 7 is denoted by “Nc.” Likewise, the electric motor/generator 8 rotates as a unit with the sun gear 4, and the rotation speed of these components 8, 4 is denoted by “Ns.” The ring gear 5, the second electric motor/generator 12, and the wheels 19 rotate in proportion with each other, to eventually provide the vehicle speed. The rotation speeds of these components 5, 12, 19 differ depending on the ratio between the number of gear teeth of the gear 15 and that of the gear 16, the speed reducing ratio of the differential gear unit 17, and the tire radius. In the following description, however, the rotation speed of the ring gear 5 will be adopted as a typical speed representing those of the components 5, 12, 19 and will be denoted by “Nr”, for the sake of simplicity and convenience.
FIG. 2 is a graph showing a relationship among the rotation speed Nc of the internal combustion engine and the rotation speeds Ns, Nr of the two electric motors MG1, MG2, which relationship is established on the basis of the principal of the planetary gear mechanism. In this graph, ρ represents the ratio of the number of gear teeth of the sun gear to that of the ring gear (ρ<1). Since Nc is determined by the rotation speed of the internal combustion engine, and Nr is determined by the vehicle speed, Ns is determined according to the following expression (1), based on the engine speed and the vehicle speed:Ns=(1+1/ρ)Nc−(1/ρ)Nr  (1)
Besides, torques at the carrier, the sun gear, and the ring gear will be denoted as Tc, Ts, and Tr, respectively. These torques are in equilibrium with each other at the following ratio;Ts:Tc:Tr=π/(1+ρ):1:1/(1+ρ)  (2)When any of these three elements, i.e., the carrier, the sun gear and the ring gear, generates or absorbs torque, torque is transferred among the elements until the above equilibrium is achieved.
In a hybrid vehicle including the drive system constructed as described above, the operations of the internal combustion engine, MG1, and MG2 are controlled by a vehicle operation control system (not shown) based on operation commands from an operator of the vehicle and the operating or running state of the vehicle. More specifically, the vehicle operation control system includes a microcomputer and is arranged to perform the following control. First, a target vehicle speed and a target wheel drive torque are calculated based on operation commands from the vehicle operator and the operating state of the vehicle detected by various sensors. At the same time, output current available at a power storage system or the quantity of electric power required for charging the power storage system are calculated based on the state of charge (SOC) of the power storage system. Using the results of these calculations, the vehicle operation control system further performs calculations to determine an appropriate operating mode of the internal combustion engine, including suspension or stop of the operation thereof, and an appropriate motor-operating/power-generating mode of each of the MG1 and the MG2. Using the results of these calculations, the vehicle operation control system controls the operations of the internal combustion engine, the MG1 and the MG2.
In the hybrid-vehicle drive system, the output shaft of the internal combustion engine is connected to the first electric motor/generator and to the wheel-drive shaft via the power distribution mechanism, and the second electric motor/generator is connected to the wheel-drive shaft, as described above. With this arrangement, as is apparent from FIG. 2, changes in the rotation speed Nc of the output shaft of the internal combustion engine, the rotation speed Nr corresponding to the vehicle speed, and the relationship between the rotation speeds Nc, Nr can be absorbed by the rotation speed Ns of the first electric motor/generator, and therefore these values Nc, Nr can be significantly changed. Thus, the hybrid-vehicle drive system does not require a transmission. More specifically, the relationship between Nc and Nr can be flexibly changed by adjusting or controlling the power distribution system, and it is therefore possible, for example, to operate the engine (Nc>0) even when the vehicle is at a stop (Nr=0), to stop the engine operation (Nc=0) while the vehicle is running forward (Nr>0), or to drive the vehicle (Nr<0) backward irrespective of whether the engine is operated or stopped (Nc≧0).
Since the rotation speed of the MG2 depends on the vehicle speed and the state of charge of the power storage system basically has no relationship with the vehicle speed, there is a great restriction to operating the MG2 as a power generator for charging the power storage system. Therefore, charging of the power storage system is carried out only by the MG1 whereas electric driving of the wheels is carried out only by the MG2. In the above-described hybrid-vehicle drive system including no transmission, therefore, the MG2 serving as a sole motor for driving the wheels needs to be large-sized in order to secure a satisfactory vehicle drive performance for generating large wheel-drive torque as needed even in a low-vehicle-speed region.
The above description will be more apparent from FIG. 3, which shows a coordinate system indicating a relationship between a required value of torque to be produced at the vehicle axles (which will be referred to as “vehicle-axle torque”) and the vehicle speed. The relationship of FIG. 3 is obtained when the internal combustion engine of the vehicle is operated at a high fuel efficiency over a wide range of vehicle speed. In FIG. 3, line A represents the limit performance of the vehicle, which represents a desired relationship between the vehicle speed and the vehicle-axle torque, and a flat region denoted by B represents the vehicle-speed versus vehicle-axle-torque performance of the internal combustion engine operating at a high fuel efficiency. The remaining region denoted by C represents the vehicle-speed versus vehicle-axle-torque performance to be provided only by the MG2. To achieve the vehicle-speed versus vehicle-axle-torque performance of FIG. 3, the MG2 is required to have a sufficiently large size so as to produce a large torque at a low rotation speed.
It appears from FIG. 3 that the depth of the region C is considerably large as compared with the region B. The imbalance between the region C and the region B may lead to an imbalance in the size among the three driving sources, i.e., the internal combustion engine and the first and second motor/generators, in particular, an imbalance in the size between the engine and the second motor/generator. In view of this point, the hybrid-vehicle drive system without a transmission as described above may be desired to be further improved.